1. Field of the Invention
The present invention relates to a multilayer capacitor and a method of manufacturing the multilayer capacitor. More particularly, the present invention relates to an improvement in the structure of an internal electrode in the multilayer capacitor.
2. Description of the Related Art
An equivalent circuit of a capacitor is represented by a circuit having C, L, and R connected in series, where the capacitance of the capacitor is denoted by C, an equivalent series inductance (ESL) is denoted by L, and an equivalent series resistance (ESR) is denoted by R.
The resonant frequency (f0) of this equivalent circuit is equal to 1/[2π×(L×C)1/2] and the equivalent circuit does not function as a capacitor in a frequency band higher than this resonant frequency. In other words, decreasing the value of L, or ESL, increases the resonant frequency and, therefore, the equivalent circuit functions as a capacitor in a higher frequency band.
For example, a decoupling capacitor, which is used in a frequency range of MHz and GHz in a power supply circuit that supplies power to the chip of a micro processing unit (MPU) of, for example, a workstation or a personal computer, requires a capacitor having a lower ESL. For example, a multi-terminal capacitor 1 shown in FIG. 12 is known as a capacitor having a lower ESL (for example, refer to Japanese Unexamined Patent Application Publication No. 11-144996).
FIG. 12 is a plan view schematically showing the multi-terminal capacitor 1.
The multi-terminal capacitor 1 has a rectangular prismatic main body 2. First external terminal electrodes 4 and second external terminal electrodes 5, having different polarities, are alternately arranged on a side surface 3 of the main body 2. In order to clearly distinguish between the first external terminal electrodes 4 and the second external terminal electrodes 5 in FIG. 12, the first external terminal electrodes 4 are shown by solid rectangles and the second external terminal electrodes 5 are shown by rectangles in outline only.
At least one pair of first and second internal electrodes (not shown) that are opposed to each other so as to generate an electrostatic capacitance is provided in the main body 2. The first external terminal electrodes 4 described above are electrically connected to the first internal electrode and the second external terminal electrodes 5 described above are electrically connected to the second internal electrode.
When current flows from the first external terminal electrodes 4 to the second external terminal electrodes 5, for example, as shown by arrows in FIG. 12, magnetic fluxes are generated whose directions are determined in accordance with the direction of the current to produce self inductance components. Since the magnetic fluxes in different directions exist in a portion in which currents having different directions flow, for example, in a portion surrounded by a broken circle 6, the magnetic fluxes are offset, resulting in a reduction in the magnetic fluxes. Accordingly, it is possible to decrease the ESL.
The decoupling capacitor in a power supply circuit that supplies power to the chip of the MPU of, for example, a personal computer is used for noise absorption and for smoothing a variation in the power supply.
FIG. 13 is a block diagram schematically showing an example of the structure in which an MPU is connected to a power supply unit.
An MPU 11 includes an MPU chip 12 and a memory 13. A power supply unit 14 supplies power to the MPU chip 12, and a decoupling capacitor 15 is connected to the circuit between the power supply unit 14 and the MPU chip 12.
When, for example, a multilayer ceramic capacitor is used, such as the decoupling capacitor described above, there is a problem in that it is difficult for the multilayer ceramic capacitor to stably operate in a higher frequency band because the multilayer ceramic capacitor is characterized by having a capacitance variation of several percent and specific temperature characteristics. Hence, multiple multilayer ceramic capacitors having different capacitances are connected to each other in parallel to produce a required impedance in a wider frequency band.
However, the multilayer ceramic capacitor has sharp impedance characteristics because it has a high Q and, therefore, the peak tends to rise in a portion where the impedance characteristics of the multiple multilayer ceramic capacitors are combined. This rise will be specifically described with reference to FIGS. 14A and 14B.
FIGS. 14A and 14B include graphs showing impedance characteristics in a case where multiple ceramic capacitors having different capacitances are connected to each other in parallel. FIG. 14A shows the respective impedance characteristics of a multilayer ceramic capacitor having a capacitance of 0.1 μF, a multilayer ceramic capacitor having a capacitance of 1 μF, and a multilayer ceramic capacitor having a capacitance of 10 μF. FIG. 14B shows combined impedance characteristics when these three multilayer ceramic capacitors are connected to each other in parallel.
As shown in FIG. 14A, the multilayer ceramic capacitors have sharp impedance characteristics. Accordingly, the peaks rise in portions where the impedance characteristics are combined to increase the impedance. As a result, there is a problem in that it is not possible to sufficiently reduce the noise in such a frequency band.
In order to resolve the problem to decrease the difference between the peaks and valleys in portions where the impedance characteristics are combined and to produce flat impedance characteristics, as shown by a broken line in FIG. 14B, it is necessary to connect a resistor to the capacitors in series. In other words, connecting the resistor to the capacitors in series allows the Q to be reduced to lower the peaks in the portions where the impedance characteristics are combined, as shown by the broken line in FIG. 14B.
However, if the resistance of the resistor connected in series is too high, the valleys rise, that is, the impedance increases while flat impedance characteristics can be produced and, therefore, it is not possible to produce the required noise absorption characteristics.
In order to inhibit the whole impedance characteristics from becoming too high while maintaining the flatness, a resistor having a minor resistance of, for example, one hundred and several tens mΩ to several hundred mΩ is required.
However, since it is difficult to mount a resistor having such a minor resistance as a part separate from the capacitor and the number of parts is increased, an increased resistance of the capacitor itself, that is, an increased ESR is considered.
It is sufficient to increase the resistance of the internal electrodes in order to increase the ESR of the multilayer ceramic capacitor, so that methods of (1) using a metal having a higher resistivity as the internal electrodes, (2) decreasing the number of layers of the internal electrodes, (3) reducing the coverage of the internal electrodes, and so on are considered. However, since the characteristics of the capacitances, etc. are greatly varied when using such methods, there is a limit to producing a resistance of one hundred and several tens mΩ to several hundred mΩ using only these methods.
Narrowing extended portions of the internal electrodes in the multilayer ceramic capacitor having a general structure to increase the ESR is known (for example, refer to Japanese Utility Model Publication No. 63-36677).
As described in Japanese Utility Model Publication No. 63-36677, decreasing the width of the extended portions of the internal electrodes to increase the ESR does not require a resistor to be provided as a part separate from the capacitor and does not have a great impact on the characteristics, such as the capacitance value, so that the decreased width of the extended portions of the internal electrodes can be considered a desirable structure for increasing the ESR.
However, when such a structure for increasing the ESR is applied to the multi-terminal capacitor described in, for example, Japanese Unexamined Patent Application Publication No. 11-144996, further decreasing the small width of the extended portions of the internal electrodes in the multi-terminal capacitor can cause the electrodes to be severed during firing. Accordingly, particularly in the multi-terminal capacitor, there is a limit to providing a smaller width in the extended portions of the internal electrode and to further decreasing the width of these parts. Consequently, it is difficult to produce a resistance of, for example, one hundred and several tens mΩ to several hundred mΩ.